1. Field of the Invention
The invention relates to a method of obtaining a saliency map from a plurality of saliency maps created from different visual quantities.
Human beings have a selective visual attention, meaning that our visual system responds primarily to a certain number of signals originating from the objects and events of our environment.
The signal that most obviously and most intuitively attracts our attention is undoubtedly the sudden appearance of an object in a scene.
2. Related Art
Finally, various studies seek to estimate, from real fixed points, the similarities of the visual characteristics attracting our gaze. As a general rule, these studies relate to the measurement of various quantities such as the variance normalized by the average brightness of the image, the entropy and the correlation between the measured fixed point and its vicinity. The main conclusions are as follows:                the contrast measurements of the fixed regions are higher than those of regions taken at random. In other words, the contrast of an area, regardless of its nature (luminance, colour, movement, texture, etc.), attracts our attention even when this area has nothing to do with the task to be carried out by the observer.        based on the correlation measurements, these studies also show that the fixed regions differ from their vicinity.        
The detection of saliency points in an image makes it possible subsequently to improve encoding and indexing methods. Obtaining saliency maps as a way of obtaining a list of the salient points of an image is described in the European patent application published under the number EP1544792, filed under the name of Thomson Licensing SA on Dec. 18, 2003.
The creation of saliency maps is relative to different visual quantities: one saliency map possibly being relative to the chromatic components, one map for each chromatic component, or even relative to the achromatic components. However, once the different saliency maps have been created, merging them can generate undesirable results.
A conventional merging method consists in normalizing the different saliency maps so as to obtain the same dynamic range. The normalization of a map C, denoted N(C), uses the overall maximum determined on the map C. The final saliency map S is then simply obtained by the following relation:CS(s)=N(N(CSA(s))+N(CSCr1(s))+N(CSCr2(s)))
with CSA(s) representing the saliency map of the achromatic component, CSCr1(s) representing the saliency map of the first chromatic component and CSCr2(s) representing the saliency map of the second chromatic component.
One advantage of this method is its simplicity. However, it does present various drawbacks:                this method does not distinguish between a saliency map having a quasi-uniform distribution and a saliency map having one or more saliency peaks;        when a number of saliency peaks are present in a saliency map, this type of merging clearly favours the highest saliency peak;        this method is very sensitive to impulse noise,        there is no interaction between maps.        